1. Field of the Invention
The invention relates to the control of document feeding devices. It relates particularly to means for controlling the transfer of documents of various lengths in a manner establishing and maintaining substantially constant gaps between documents while delivering the documents at a constant speed.
2. Description of the Prior Art
It is highly desirable that document feeding devices supply documents at a constant speed and with constant spacing, or gaps, between them in order to enable sorting equipment downstream of the feeder to provide improved throughput of documents.
Some prior art document feeders have depended on stopping and starting rollers at various times to control spaces between documents. That procedure has been hard on the documents and has tended to wear the mechanisms of the feeders excessively as well as to be inefficient in the use of energy. For high-speed, short-gap feeding systems, the large accelerating forces required to speed up or slow down such an intermittently driven system are unrealistically high.
Among the known prior art devices is a "Constant Spacing Document Feeder" disclosed in U.S. patent application Ser. No. 111,804 filed Jan. 14, 1980 as a continuation of application Ser. No. 942,469 (now abandoned) which uses an approximation method for gap control in which the feeding elements operate between two substantially different speeds. While that system serves to eliminate the need to bring the feeding elements to a complete stop, it typically requires changes in speed twice for every document, resulting in high stresses and high energy consumption. It will be noted also that while that method has the effect of substantially cutting down on gap variation, it is not sufficiently exact for some purposes and does not provide the level of gap control possible with the present constant spacing feeder. Furthermore, documents processed by that prior art apparatus are often released to the constant speed portion of the system at speeds other than the transport speeds so a secondary uncontrolled acceleration occurs after there has been a gap adjustment, which adds further to undesirable variations in gap length.